1. Field of the Invention
The present invention relates to power supply devices for high frequency power amplifying circuits. In particular, the present invention relates to a power supply device that enhances power efficiency of a high frequency power amplifying circuit and also relates to a high frequency power amplifying apparatus that includes such a power supply device.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2006-514472 describes a configuration for enhancing the efficiency of a high frequency amplifying circuit, in which a power supply voltage of the high frequency amplifying circuit is made to follow an amplitude change (envelope) of a high frequency signal to be amplified. The frequency of the amplitude change of the high frequency signal in this case is not a carrier wave frequency (e.g., 1 GHz to 2 GHz in cellular phones) but is a frequency (approximately 10 MHz to 100 MHz at the maximum) of the amplitude change of the carrier wave. An example of a low frequency amplifying circuit is a class-D amplifying circuit (digital amplifier), but in a power supply device for a high frequency power amplifying circuit, a frequency band to be handled is high, and thus high speed switching is required.
FIG. 22 is a block diagram of a high frequency power amplifying circuit 50 described in Japanese Unexamined Patent Application Publication No. 2006-514472. The high frequency power amplifying circuit 50 illustrated in FIG. 22 includes an envelope detecting circuit 54, a supply voltage selecting circuit 56, a power supply voltage adjusting circuit 58, and a high frequency amplifying circuit 52. The envelope detecting circuit 54 detects an envelope of a high frequency signal (RFIN), and the supply voltage selecting circuit 56 selects a voltage source (V1 through V4) so that a power supply voltage follows the envelope of the high frequency signal (RFIN). The power supply voltage adjusting circuit 58 adjusts the waveform of the selected voltage and supplies the result to the high frequency amplifying circuit 52 as the power supply voltage. Through this, the high frequency amplifying circuit 52 amplifies the high frequency signal (RFIN) and outputs a high frequency signal (RFOUT).
The high frequency power amplifying apparatus described in Japanese Unexamined Patent Application Publication No. 2006-514472, however, employs a system in which a plurality of voltage sources are prepared and a voltage source is selected so as to cause the power supply voltage to follow the envelope of the high frequency signal. Therefore, a complex circuit and extremely complex control are required to repair a voltage difference (waveform steps) occurring when the plurality of voltage sources are switched. If this repair of the waveform steps is incomplete, distortion in the waveform increases, and an adjacent channel leakage power ratio (ACPR) in a communication system increases in turn, which negatively affects frequencies outside a stipulated range and consequently leads to a problem in that the high frequency power amplifying apparatus cannot be applied to a product having a strict permissible frequency band.